1. Technical Field
The present invention relates in general to an improved fabrication technique and, in particular, to an improved apparatus and method of making a single-crystal structure through free-form fabrication techniques.
2. Description of the Related Art
Metal objects are currently produced by thermomechanical processes which include casting, rolling, stamping, forging, extrusion, machining, and joining operations. Multiple steps are required to produce a finished article. These conventional operations often require the use of heavy equipment and molds, tools, and dies. For example, a typical process sequence required to form a small cylindrical pressure vessel might include casting an ingot, heat treating and working the casting to homogenize it by forging or extrusion or both, then machining a hollow cylinder and, separately, end caps from the worked ingot and, finally, welding the end caps to the cylinder. This invention provides a method for forming such an article in a single operation and using less equipment. An article formed by laser deposition is relatively free of internal stresses in comparison to an article formed by welding one or more components together. Also, joining components by means of welding requires equipment just for the single step of joining and time to set up the equipment to do the job.
Conventional production methods are subtractive in nature in that material is cut away from a starting block of material to produce a more complex shape. Subtractive machining methods are deficient in many respects. Large portions of the starting material are reduced to waste in the form of cuttings. These methods produce waste materials, such as metal cuttings, oils, and solvents, which must be further processed for purposes of reuse or disposal. The articles produced are contaminated with cutting fluids and metal chips. They require cutting tools which wear and must be periodically reconditioned and ultimately replaced. Fixtures for use in manufacturing must be designed, fabricated, and manipulated during production. When a part is unusual in shape or has internal features, machining is more difficult. Choosing the machining operations to be used and the sequence of operations requires a high degree of experience. A number of different machines are needed to provide capability to perform the variety of operations which are often required to produce a single article. Sophisticated machine tools require a significant capital investment and occupy a good deal of space. Use of the invention in place of subtractive machining provides solutions to these problems and disadvantages. The inventive process may be characterized as additive in nature. The raw material which does not become part of an article is easily collected and re-used without processing. There is no need to dispose of waste liquids and metal cuttings and the articles produced are not contaminated by these materials. Fixtures and cutting tools are not required. All work needed to produce an article is accomplished using a computer workstation and a single production station.
Another difficulty with conventional machining techniques is that many objects must be produced by machining a number of parts and then joining them together. Producing parts separately and joining them requires closetolerance machining of matching parts, provision of fastening means, such as threaded connections, and welding together of components. These operations involve a significant portion of the cost of producing an article, as they require time for design and production as well as apparatus for performing them.
Typically, items formed using direct-deposition methods have polycrystalline grain structures that grow with their crystal lattices in various directions. This occurs due to nucleation at multiple sites, and the result is growth of misaligned crystal lattices that extend toward each other, with the lattices abutting at the grain boundaries. While this may be allowable in some applications, the grain boundaries allow for the “creep” mechanism of deformation to occur in applications where the material experiences elevated temperatures and high-stress and/or stress for long durations. Conditions such as these are common for applications such as turbine blades in jet engines.
In conventional directional solidification, a mold is filled with molten metal and slowly cooled from one direction. This forms a two-dimensional, formal gradient which allows the solid crystal to grow in one direction, thus maintaining its preferred orientation. Single-crystal materials also may be made through a variety of other methods including gradient cooling in investment castings (floating zone method), Czochralski crystal pulling, etc. Although each of these methods is workable, they do have disadvantages. For example, some of these methods require expensive mold cavities, fixtures, and tightly-controlled and/or segmented furnaces to produce some components. Still other prior art methods have additional disadvantages. See, e.g., U.S. Pat. No. 5,837,960 to Lewis, et al; U.S. Pat. No. 5,437,820 to Brotz; U.S. Pat. No. 6,046,426 to Jeantette, et al; U.S. Pat. No. 4,599,133 to Miyao, et al; U.S. Pat. No. 5,998,097 to Hatakeyma, et al; and U.S. Pat. No. 5,960,853 to Sterrett, et al. The components utilized by these inventions can add very significant cost and lead times for producing the end products. Thus, an improved apparatus, system, and method of producing single-crystal structures that avoids or eliminates many of the disadvantages of the prior art is needed and would be desirable.